This invention relates to a liquid crystal display unit and, more particularly, to a wide viewing angle liquid crystal display having an optical compensator and an optical diffuser.
The liquid crystal display has various attractive features such as light, thin and low in power consumption, and find a wide variety of application. A flat display panel is a typical consumer product of the liquid crystal display, and forms an essential part of an office machine, portable television set for vehicles or a navigation system for vehicles.
The liquid crystal display used in the vehicles is expected to have a wide viewing angle, because the liquid crystal display is usually placed to be close to the driver. An image produced on the liquid crystal display has a viewing angle dependency. The image is assumed to be recognized as a black configuration by a person sitting in front of the liquid crystal display. However, when the person is laterally moved over a critical angle, the image whitens, and the person feels it unclear. For this reason, the liquid crystal display for vehicles is expected to have the wide viewing angle characteristics.
FIG. 1 illustrates a standard liquid crystal display. The prior art liquid crystal display largely comprises a surface light source 1 and a liquid crystal panel 2. The surface light source 1 is implemented by a cold-cathode tube fluorescent lamp associated with an optical guide plate, and illuminates the liquid crystal panel 2. The cold-cathode tube fluorescent lamp is assumed to be a line light source, and the light is incident into an input end surface of the optical guide plate. The light repeats the total reflection in the optical guide plate, and diffusing elements in the optical guide plate cause the light la to uniformly illuminate the liquid crystal panel 2. If the light 1a is less uniform, a diffusing sheet is further attached to the output end surface of the optical guide plate.
The liquid crystal panel 2 includes a pair of transparent substrate structures 2a and 2b spaced apart from each other by a sealing layer 2c and twisted nematic liquid crystal 2d filling the gap between the transparent substrate structures 2a and 2b. The twisted nematic liquid crystal 2d has a twisting angle of about 90 degrees.
The transparent substrate structure 2a has a plurality of pixel electrodes arranged in matrix, a plurality of thin film transistors selectively connected between the pixel electrodes and image signal lines and gate control lines selectively connected to the gate electrodes of the thin film transistors, and these components are formed on and over a transparent glass substrate. On the other hand, the transparent substrate structure 2b has a transparent counter electrode and color filters formed on and over a transparent glass substrate.
The liquid crystal panel 2 further includes polarizing plates 2e/2f attached to the outer surfaces of the transparent substrate structures 2a/2b, respectively. When the standard liquid crystal display is designed to be normally white mode, the polarizing plates 2e/2f have transmission axes angularly spaced at 90 degrees. The pixel electrodes, the twisted nematic liquid crystal and the counter electrode form a matrix of liquid crystal capacitors, and the liquid crystal capacitors serve as a matrix of pixels where an image is produced
When an image 3a is produced on a screen 3b, the counter electrode is biased to a certain constant potential level, and image-carrying signals sequentially bias the pixel electrodes to potential levels representing the gradations by controlling the thin film transistors. The droplets of liquid crystal droplet 2d rise at different angles depending upon the potential difference between the pixel electrodes and the counter electrode, and the pixels have a certain dispersion of transmittance so as to produce the image 3a on the screen 3b. 
When a person sees the image 3a in front of the screen 3b, the person clearly recognizes the image 3a on the screen 3b. However, the liquid crystal 2d has an anisotropic refractive index. If the image is obliquely seen, an inversion of gradation or deterioration of contrast ratio take place, and the person hardly discriminates the image 3a on the screen 3b. The inversion of gradation is the phenomenon where an image recognized by a person is produced in a gradation inverted from a gradation of an image intended to be produced on the screen. The contrast ratio is a ratio between white luminance and black luminance. In general, when the person is downwardly moved to a position at 5 degrees with respect to a vertical line to the screen 3b, the inversion of gradation takes place. If the angle exceeds 30 degrees toward an upper portion of the screen and 50 degrees toward a lower position of the screen, the contrast ratio is decreased to 10 or less.
Thus, the anisotropy of the refractive index makes the viewing angle of the liquid crystal display narrow, and various technologies have been proposed for wide viewing angle characteristics. One of the techniques is disclosed in Japanese Patent Publication of Unexamined Application No. 7-120619, and FIGS. 2 and 3 illustrates an index ellipsoid of an optically anisotropic element and a liquid crystal panel using the optically anisotropic film both disclosed in the Japanese Patent Publication of Unexamined Application. The liquid crystal panel shown in FIGS. 2 and 3 is hereinbelow referred to as xe2x80x9cfirst prior artxe2x80x9d.
The optically anisotropic film 10 has the principal indices of refraction nx, ny and nz where nx greater than ny greater than nz. As shown in FIG. 2, nx is on the optically anisotropic film 10, and ny and nz decline with respect to a rotating axis aligned with the direction of nx. The principal indices of refraction are represented by an index ellipsoid. The optically anisotropic film 10 is effective against reduction of contrast due to a double refraction inherent in liquid crystal.
Using a pair of optically anisotropic films 10a/10b, the first prior art liquid crystal panel comprises a liquid crystal cell 11 provided between the optically anisotropic films 10a/10b and polarizing layers 12b/12b, and the polarizing layers 12a/12b are spaced from each other by a gap where the liquid crystal cell 11 and the optically anisotropic films 10a/10b are inserted. The liquid crystal cell 11 is analogous to the prior art standard liquid crystal panel, and includes a bottom transparent substrate 11a, transparent electrodes 11b formed on the bottom transparent substrate 11a, a top transparent substrate 11c spaced from the bottom transparent substrate 11a by a spacer 11d, a transparent electrode 11e formed on the lower surface of the top transparent substrate 11c and liquid crystal 11f filling the gap between the bottom transparent substrate 11a and the top transparent substrate 11c. 
Although the optically anisotropic films 10a/10b suppress the reduction of contrast due to the double refraction of the liquid crystal, the optically anisotropic films 10a/10b can not sufficiently suppress the inversion of gradation. When twisted nematic liquid crystal is used for the first prior art, an image is unintentionally colored.
Another improvement is disclosed in Japanese Patent Publication of Unexamined Application No. 7-159614, and is hereinbelow referred to as xe2x80x9csecond prior artxe2x80x9d. FIG. 4 illustrates a polarization of light LO obliquely incident onto the second prior art. The second prior art comprises a liquid crystal cell 20, an optically anisotropic element 21 and a pair of polarizing plates 22a/22b, and liquid crystal LC fills a gap in the liquid crystal cell 20.
When the light is obliquely incident onto the polarizing plate 22a, lineally polarized light Li takes place, and proceeds toward the liquid crystal cell 20. The liquid crystal LC converts the linearly polarized light Li to elliptically polarized light L2 due to the an isotropy of refractive index of the liquid crystal LC.
If the elliptically polarized light L2 directly proceeds to the polarizing plate 22b, the polarizing plate 22b can not appropriately block the elliptically polarized light L2, and leakage takes place. However, the optically anisotropic element 21 is provided between the liquid crystal cell 20 and the polarizing plate 22b. The optically anisotropic element 21 introduces a retardation, and converts the elliptically polarized light L2 to linearly polarized light L3. Even if the light is obliquely incident onto the second prior art, the second prior art provides a transmittance to the obliquely incident light equal to that of light normally incident thereto. Thus, the optically anisotropic element 21 eliminates the viewing angle dependency from the second prior art.
The optically anisotropic element 21 is formed of optically negative uniaxial crystal. When the refraction indices in the major three axes are called as n-alpha, n-beta and n-gamma in the order of magnitude, i.e., from the smallest value toward the largest value, the refraction indices have the following relation, n-alpha less than n-beta less than n-gamma. Thus, the optically anisotropic element 21 has the smallest refractive index in the direction of optical axis. The optical axis is not on the incident surface of the optically anisotropic element, nor is matched with the normal line to the incident surface. However, the second prior art can not sufficiently suppress the inversion of gradation.
Yet another improvement is disclosed in Japanese Patent Publication of Unexamined Application No. 6-82776, and FIG. 5 illustrates the prior art liquid crystal display panel disclosed in the Japanese Patent Publication of Unexamined Application. The prior art liquid crystal display panel is hereinbelow referred to as xe2x80x9cthird prior artxe2x80x9d.
The third prior art comprises a liquid crystal display panel 31 sandwiched between polarizing plates 32a and 32b, and an optical diffusion plate 33 is attached to the polarizing plate 32b. A light source (not shown) is provided at the back of the polarizing plate 32a, and illuminates the liquid crystal display panel 31 through the polarizing plate 32a. The light is incident onto the liquid crystal display panel 31, and carries an image from the liquid crystal display panel 31 through the polarizing plate 32b to the optical diffusion plate 33. The optical diffusion plate 33 diffuses the image carrying light, and averages the contrast ratio over the image forming screen. As a result, the contrast ratio is improved at a large viewing angle.
The optical diffusion plate 33 is formed of acrylic resin or polyvinylalcohol containing white pigment or having rough surface. However, the optical diffusion plate 33 lowers the contrast to a person who sees an image from the vertical direction to the image forming screen, because the light obliquely leaked through a black image is reflected toward the front direction. Moreover, the optical diffusion plate 33 whitens the image due to the irregular reflection.
As will be understood from the foregoing description, the prior art standard liquid crystal display panel and the first to third prior art liquid crystal display panels encounter the following problems.
The prior art standard liquid crystal display panel is narrow in viewing angle, and the gradation is destroyed due to the difference in retardation, i.e., the product of the difference delta-n between the refractive index ne for an extraordinary ray and the refractive index n0 for an ordinary ray and the liquid crystal gap d. When the viewing point is widely deviated from the normal line to the image forming screen, the destruction of the gradation becomes more serious.
The first and second prior art liquid crystal display panels can suppress the reduction in contrast ratio by virtue of the optically anisotropic elements. However, the inversion of gradation still takes place, and images are unintentionally colored when the twisted nematic liquid crystal is used.
The third prior art liquid crystal display panel lowers the contrast at the vertical position to the image forming screen due to the averaging of the contrast over the image forming screen, and whitens the image due to the irregular reflection. Moreover, the optical diffusion plate reflects the light from the outside thereof, and makes the visibility poor.
It is therefore an important object of the present invention to provide a liquid crystal display panel which improves the viewing angle and is prevented from the inversion of gradation, the unintentionally colored image and poor visibility.
To accomplish the object, the present invention proposes to compensate the anisotropy of refractive index of liquid crystal and diffuse image carrying light after the optical compensation.
In accordance with the present invention, there is provided a liquid crystal display unit comprising: a light source generating first light proceeding along an optical path; polarizing layers provided on the optical path, and spaced from each other; a liquid crystal panel provided between the polarizing layers, and including a liquid crystal layer for producing second light carrying an image from the first light and having a first anisotropic index of ellipsoid with respect to one of a normal line to an incident surface of the liquid crystal panel and an oblique line declining from the normal line; an optically anisotropic layer provided on the optical path, and having a second anisotropic index of ellipsoid converse to the first anisotropic index of ellipsoid with respect to the aforesaid one of the normal line and the oblique line; and an optically diffusing layer provided on the optical path for averaging the intensity of the second light passing through the optically anisotropic layer.
The liquid crystal display unit may further comprise a condenser layer.